Known acoustic microscopes are used for imaging structures such as integrated circuit (IC) structures. The spatial resolution, w, of an acoustic microscope is given by:
  w  =      0.51    ⁢          ϑ              f        ·        NA            where θ is the speed of sound in the coupling medium, f is the frequency of the acoustic/ultrasonic wave, and N.A. is the numerical aperture of the lens. For a frequency of 1 GHz, the nominal spatial resolution attainable is approximately 1.5 μm. Further, the acoustic microscope has two other major roadblocks in getting high resolution: (1) impedance mismatches and coupling fluid attenuation that is proportional to f2. Higher resolution alternatives for nondestructive mechanical imaging include the atomic force microscope (AFM) or scanning probe microscope (SPM) platforms. A few examples include: force modulation microscopy (FMM) as described by P. Maivald, H. J. Butt, S. A. C. Gould, C. B. Prater, B. Drake, J. A. Gurley, V. B. Elings, and P. K. Hansma in Nanotechnology 2, 103 (1991); ultrasonic-AFM as described by U. Rabe and W. Arnold in Appl. Phys. Lett. 64, 1423 (1994); and ultrasonic force microscopy (UFM) as described by O. V. Kolosov, K. Yamanaka in Jpn. J. Appl. Phys. 32, 1095 (1993); by G. S. Shekhawat, O. V. Kolosov, G. A. D. Briggs, E. O. Shaffer, S. Martin and R. Geer in Nanoscale Elastic Imaging of Aluminum/Low-k Dielectric Interconnect Structures, presented at the Material Research Society, Symposium D, April 2000 and published in Materials Research Society Symposium Proceedings, Vol. 612 (2001) pp. 1; by G. S. Shekhawat, G. A. D. Briggs, O. V. Kolosov, and R. E. Geer in Nanoscale elastic imaging and mechanical modulus measurements of aluminum/low-k dielectric interconnect structures, Proceedings of the International Conference on Characterization and Metrology for ULSI Technology, AIP Conference Proceedings. (2001) pp. 449; by G. S. Shekhawat, O. V. Kolosov, G. A. D. Briggs, E. O. Shaffer, S. J. Martin, R. E. Geer in Proceedings of the IEEE International Interconnect Technology Conference, 96-98, 2000; by K. Yamanaka and H. Ogiao in Applied Physics Letters 64 (2), 1994; by K. Yamanaka, Y. Maruyama, T. Tsuji in Applied Physics Letters 78 (13), 2001; and by K. B. Crozier, G. G. Yaralioglu, F. L. Degertekin, J. D. Adams, S. C. Minne, and C. F. Quate in Applied Physics Letters 76 (14), 2000. Each of these techniques is traditionally sensitive to the static elastic properties of the sample surface.
Recent developments in atomic force microscopes have involved the application of ultrasonic frequency (MHz) vibrations to the sample under study and non-linearly detecting of the deflection amplitude of the tip at the same high frequencies. With this arrangement, which is commonly identified as an ultrasonic force microscope, the ultrasonic frequencies employed are much higher than the resonant frequency of the microscope cantilever. The microscope exploits the strongly non-linear dependence of the atomic force on the distance between the tip and the sample surface. Due to this non-linearity, when the surface of the sample is excited by an ultrasonic wave, the contact between the tip and the surface rectifies the ultrasonic vibration, with the cantilever on which the tip is mounted being dynamically rigid to the ultrasonic vibration. The ultrasonic force microscope enables the imaging and mapping of the dynamic surface viscoelastic properties of a sample and hence elastic and adhesion phenomenon as well as local material composition which otherwise would not be visible using standard techniques at nanoscale resolution.
The drawback of ultrasonic microscopy is that it measures only the amplitude due to ultrasonically induced cantilever vibrations. Moreover, where the sample is particularly thick and has a very irregular surface or high ultrasonic attenuation, only low surface vibration amplitude may be generated. In such circumstances the amplitude of vibration may be below the sensitivity threshold of the microscope in which case measurement is impossible. Moreover, none of the above mentioned techniques measures with high resolution the acoustic phase, which is very sensitive to subsurface elastic imaging and deep defects identification which are lying underneath the surface, without doing any cross sectioning of the samples.
Out-of-plane vibrations created by non-linear tip sample interaction make a very hard elastic contact with the sample surface. Ultrasonic force microscopy (UFM) uses the same method except for a amplitude component rather than a phase contrast. If non-linearity is present in the system, most of the phase contrast will come from the surface and not from a surface/sub-surface phase contrast. Additionally, non-linear tip sample interaction may not provide results for soft materials. Furthermore, in UFM, high mechanical contrast may be acquired with little sub-surface contrast.